This volume contains the proceedings of the Sixth International Symposium
on Games, Automata, Logic and Formal Verification (GandALF 2015). The symposium took place
in Genoa, Italy, on the 21st and 22nd of September 2015.

The GandALF symposium was established by a number of Italian computer scientists interested in
mathematical logic, automata theory, game theory, and their applications to the specification,
design, and verification of complex systems. Its aim is to provide a forum where people from
different areas, and possibly with different backgrounds, can fruitfully interact. Even though
the idea of the symposium emerged within the Italian research community, the event has a truly
international nature, as witnessed by the composition of the conference committees and by the
country distribution of the submitted papers.

The programme committee received 22 submissions and
selected 13 of them for presentation at the symposium. Each paper was revised by at
least three referees, and the selection was based on originality, quality, and relevance to
the topics of the call for papers. The scientific program contained preentations on algorithmic
game theory, automata theory, formal verification, and modal and temporal logics. Additionally,
three invited talks were given by Sven Schewe (University of Liverpool), Nathalie Bertrand (INRIA Rennes),
and Viktor Vafeiadis (Max Planck Institute for Software Systems, Kaiserslautern); their titles
and abstracts can be found below.

We wish to express our thanks to the authors
who submitted extended abstracts for consideration, the speakers and invited speakers,
the programme committee members (also listed below), and the additional
reviewers for their excellent work.

We thank the Gruppo Nazionale per il Calcolo Scientifico (GNCS/Indam), the Italian Chapter of EATCS,
the University of Genoa, and in particular its Dipartimento di Informatica, Bioingegneria, Robotica ed Ingegneria dei
Sistemi (DIBRIS), for their help in the organization of the event. We also thank the EasyChair organization
for supporting all the tasks related to the selection of contributions, and EPTCS and arXiv for hosting the proceedings.

We would like to extend special thanks to the organizing committee,
composed by Giorgio Delzanno, Elena Zucca, Alessandro Solimando, Daniela Briola, Andrea Corradi,
and Federico Frassetto, for setting up an attractive scientific and social program, and
to the General Chair of GandALF, Margherita Napoli, for her constant support.

Parity games are simple two player games played on a finite arena that have all that it takes to attract the attention of researchers: it is a simple problem which is hard to analyse - a pocket version of P vs. NP. Parity games are known to be in UP, CoUP, PPAD, and PLS (and some more classes), but whether or not they are in P has proven to be a rather elusive question. What is more, when you work with them, you will have the constant feeling that there is a polynomial time solution just around the corner, although it dissolves into nothingness when you look more closely. This talk is about the beauty of these games, the relevant algorithmic approaches and their complexity and development over time. But be careful: they are addictive, don't get hooked!

We study verification problems for a model of network with the following characteristics:
communication is performed through broadcast with adjacent neighbours, entities can change their internal state probabilistically, and reconfiguration of the communication topology can happen at any time. For such reconfigurable probabilistic broadcast networks, the semantics is given in terms of a Markov decision process, combining non-determinism with probabilities. Importantly, although the number of nodes is finite and fixed during a computation, it is unknown and represented by a parameter. We are interested in parameterized qualitative problems like whether independently of the number of nodes, there exists a resolution of the non-determinism such that a configuration exhibiting an error state is almost surely reached. We show that all variants of this qualitative reachability problem are decidable. Some proofs are based on solving a infinite-state 2-player games with parity and safety objectives, described as reconfigurable broadcast networks.
This is based on a joint work with Paulin Fournier and Arnaud Sangnier.

In this talk, I will introduce weak memory consistency models and the challenges it poses on software verification. I will then focus on the release-acquire memory model, an important fragment of the C/C++ memory model, and present a few verification results that allow us to reason about concurrent programs under the release-acquire model.